The present invention relates to controlling the arc bowing normally encountered in arc discharge lamps. When a discharge lamp is operated with the arc tube in a position other than essentially vertical, and in particular when the arc tube and operating arc are in a horizontal or near horizontal position, the arc discharge does not remain along the central axis of the arc tube during continued operation, but bows upward within the arc tube due to convection. This arc bowing can and often does result in early lamp failure through excessive heating of a particular portion of the arc tube wall.
It is well known that high-pressure mercury-vapor lamps operated at high power levels in a horizontal burning position tend to have a hot portion of the arc tube at the center top section thereof. Electromagnetic means have been used to deflect the arc discharge down into a more central portion within the arc tube by directing the magnetic field transverse to the arc current direction, wherein the interaction of the arc discharge current and magnetic field results in a downward force on the arc.
U.S. Pat. No. 3,562,583, issued to Zollweg and Burnham on Feb. 9, 1971 describes a magnetically rotating, constricted arc discharge device having electromagnetic means to rotate a resultant magnetic flux generally transverse to the arc discharge to cause the arc discharge to rotate about the longitudinal axis of the arc tube and thereby avoid destructive arc bowing.
The use of a separate electromagnet consumes extra power, takes up a great deal of space, and can block a large amount of the light emitted from the arc tube. Permanent magnets can also be used, but they also are bulky and generally block a significant amount of light.